Inverters for Single-phase Grid Connected Photovoltaic Systems - An Overview Martina Calais Johanna Myrzik2 Ted Spoone? Vassilios G. Agelidis4

School of Engineering, Murdoch University, Murdoch WA 6150, Australia Technical University of Eindhoven, The Netherlands

School of Electrical Engineering and Telecommunications, University of New South Wales, Australia * Inter-University Centre for Economic Renewable Power Delivery (CERPD), University of Glasgow, U.K.

Abstract - An overview on recent developments and a summary of the state-of-the-art of inverter technology in Europe for single- phase grid-connected Photovoltaic (PV) systems for power levels up to 5 k W is provided in this paper. The information includes details not only on the topologies commercially available but also on the switching devices employed and the associated switching frequencies, efficiency, price trends and market share. Finally, the paper outlines issues associated with the development of relevant international industry standards affecting PV inverter technology.

I. INTRODUCTION

The continuing decrease of the cost of the PVs, the ad- vancement of power electronic and semiconductor technology and favourable incentives in a number of industrial countries in general had a profound impact on the commercial accep- tance of grid connected PV systems in the recent years. A core technology associated with these systems remains the inverter, which has evolved to quite mature technology offering a num- ber of advantages to customers that were not possible many years ago. The technology has changed from line commutated inverters to switch mode ones mainly due to the availability of high frequency fully-controlled switching devices.

Most inverters on the market in the mid 1990s were self or line commutated central inverters, with DC power ratings above 1 kW, suitable for PV system configurations with sev- eral strings in parallel as shown in Fig. 1. During the 1000 Roofs Program, a subsidy program of the German Federal and State Governments (which was accompanied by an extensive measurement and analysis program [ 1,2]) the disadvantages of central inverters became apparent. These include complete loss of generation during inverter outages and losses due to the mis- match of strings. String inverters, which are designed for a

Rated ~ t c u r r e m w) Fig. 1. PV invertem available in 1994 and 2002 shown versus DC voltage and

DC current ratings.

system configuration of one string of PV modules (see Fig. 1) have since become more popular.

Module integrated or module oriented PV inverters with rated power below 500 W can be classified as a third group of PV inverters beside central and string inverters (see Fig. 1). They have been available on the market since the mid 1990s and their modularity allows for small, simple systems which can easily be expanded by paralleling more AC-modules. The concept allows mismatch and losses due to shading to be reduced even further than with string inverters.

A recent development is the multi string PV inverter con- cept, where several DC to DC converters are connected to one central inverter. Unlike the string inverter concept, the multi string inverter requires only one central inverter for all supervi- sory and protection functions. [3,4,51

The topologies used in the different PV system concepts are described and discussed in Section I1 of this paper. Information on employed switching devices and switching frequencies, ef- ficiencies, price trends and market shares is included as well. Section 111 provides some background information on the de- velopments on standards affecting this technology, Finally con- clusions are summarised in Section IV.

11. INVERTER TOPOLOGIES

An inverter has to fulfill three functions in order to feed energy from a PV array into the utility grid: 1. To shape the current into a sinusoidal waveform; 2. To invert the current into an AC current, and 3. if the PV array voltage is lower than the grid voltage, the PV array voltage has to be boosted with a further element. The way these three functions are sequenced within an inverter design determines the choice of semiconductors and passive components and consequently their losses, sizes and prices. This section discusses different inverter topologies available on the European market and gives an overview on their market shares, efficiencies and price developments over the last decade.[6,7, 8,9, 10, 11,3, 12, 131

A. Central inverters Based on drive system technology the first PV inverters at

the end of the 1980s were line commutated inverters (see Fig. 2(b)) with power ratings of several kilo watts. Although these topologies are robust, highly efficient and cheap, their major drawbacks are a power factor between 0.6 and 0.7 [14],

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which has to be compensated with special filters as well as high harmonic content in-the output current. Due to the rapid developments in the semiconductor device industry, thyristors have been increasingly replaced by BJT's, MOSFET's or IGBT's. Currently employed switching devices in PV inverters are shown in Fig. 4. Today central inverters are mostly self- commutated inverters in the power range above 2 kW. Their topologies without and with transformer are shown in Fig. 2(a) and 3(a). They are composed of a PWM full bridge, switching at high frequencies (> 16 kHz) which shapes and inverts the input current into an AC current. Most of the bridges use IGBT's or a combination of IGBT's and MOSFET's (see Fig. 4). This concept is a well known, robust, efficient and cheap technology which provides high reliability and low price per watt. Their efficiencies are lower than in line commutated concepts (see Fig. 9) due to the high switching frequencies of

Fig. 3(b) shows a magnetic coupled inverter [15] available on the American market. The inverter consists of three conventional single-phase full bridges each with their mid- points connected to the primary winding of a transformer. The secondary windings of the transformers are connected in series and the turns ratios of the transformers are chosen as multiples of each other. Generally, an inverter of this type having n primary transformer windings is capable of generating 3n combinations of different voltages across the secondary transformer windings and synthesises the sine wave by means of a stepped waveform (not by means of PWM). The advantage of this circuit is the relatively accurate replica of a sine wave accomplished with low switching frequencies and a cheap and robust full-bridge. A major drawback of the circuit, however, is the need for three transformers.

The disadvantages of all central inverter topologies are found in the system configuration: 1. The required DC wiring increases costs and decreases safety;

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2. There are no means of independently operating sections of the PV array at their maximum power point (MPP). Mismatch between sections (e.g. caused by partial shading) may there- fore significantly reduce the overall system output. 3. Due to the high power range an extension or a flexible system design cannot be realised. These drawbacks can be overcome with module integrated or oriented inverters and with string inverters.

B. Module integrated or module oriented inverters These inverters are operating directly on one or several

PV modules below 500 W. The PV array voltage is generally between 30 - 150 V. These low voltage levels require a voltage adjustment element, which allows for a variety of topologies. Topologies with transformer are shown in Fig. 3(a) and Fig. 5. Using a line frequency transformer is advantageous since low voltage MOSFET's can be used for the PWM high frequency

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switched bridge. Low voltage MOSFETs which are widely used in large quantities for automotive applications are cheap semiconductor devices. Furthermore the whole control system can be realised on the low voltage side and this topology is also suitable for high current PV modules. However, some inverter companies follow high-frequency transformer concepts in order to reduce the magnetic components and costs and an example topology is shown in Fig. 5. In order to reduce the switching losses on the high voltage side the push-pull converter boosts the voltage to grid level and shapes the current waveform as well. A full bridge switched at line frequency is used in a second converter stage as an unfolding I inverting stage. Both converters in series reduce the efficiency and make the control more complex.

Fig. 6 shows a third topology available on the market, which avoids a transformer in order to reduce magnetic components and to increase efficiency. This topology can be used in several European countries e.g. Germany. Other countries require a transformer. While using a boost converter to boost the low PV voltage, shaping and inverting of the output current have to be done in the second converter stage at high voltage level.

One manufacturer produces module integrated inverters with MOSFETs switching at 400 and 800 Wz. The possibly resonant topology is unknown.

Module integrated and module oriented inverters provide the highest system flexibility. Each module has its own MPP tracking, furthermore, no DC wiring is required. Their plug and play characteristic is attractive, as is their ability to provide a complete PV system at low (plus rapidly decreasing) investment cost. However, the main disadvantage of these

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inverters is the high cost per watt. Further disadvantages are the difficult and expensive replacement in case of inverter fault and special safety requirements (depending on the country) may increase the system price.

C. String inverters The string inverter is capable of combining the advantages

of both central and module integrated inverter concepts with little tradeoffs. A number of PV modules connected in series form a string up to 2 kW (Fig. 1). In this power range the PV array (string) voltage can be between 150-450 V. Various topologies (for example those shown in Figs. 2, 3(a), 5 and 6) can be used for this concept. Depending on the power and voltage ratings, IGBTs and MOSFETs are used at switching frequencies between 16 and 32 kHz. The advantages are that these topologies are used in a higher power range, which decreases the price per watt and that the system efficiency is I-3% higher than in systems with central inverters [16].

D. Multi string inverters Recent developments in subsidy programs in Germany are

forcing companies to reduce inverter costs by approximately 20% within 5 years [5]. In order to achieve this goal a new inverter concept (see Fig. 7) has been developed to combine the advantage of higher energy yield of a string inverter with the lower costs of a central inverter. Lower power DCDC converters are connected to individual PV strings. Each PV string has its own MPP tracker which independently optimises the energy output from each PV string. To expand the system within a certain power range only a new string with a DCDC converter has to be included. All DCDC converters are con- nected via a DC bus through a central inverter to the grid. The central inverter is a PWM inverter based on the well-known and cheap IGBT technology already used in drive systems and includes all supervisory and protection functions. Depending

1997

on the size of the string the input voltage ranges between 125 to 750 V. The inverter has a maximum power rating of 5 kW.

Fig. 1 summarises the DC current and DC voltage ratings of the discussed inverter concepts available in 1994 and 2002 and demonstrates the trend towards the string inverter concept.

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E. New trends One of the common drawbacks of a single-phase inverter is

the need for large electrolyte capacitors at the input of the in- verter. Electrolyte capacitors significantly affect the efficiency, cost and lifetime of the inverter and some manufacturers have started to develop small three-phase inverters in order to reduce the electrolyte capacitor size.

For bigger PV plant applications the new team concept provides a better utilisation of the inverters at low load. In this concept the string technology will be combined with the master - slave concept. At low solar irradiation all strings are connected in parallel to one inverter while the other inverters are disconnected. At increasing irradiation the PV array is divided into sub-arrays which are connected to different independently operating string inverters. An increase of the system efficiency of 2% is expected. The concept is particularly interesting for countries with high energy conversion during part load operation.

E Eficiencies 8 shows the average maximum efficiencies of the

different topologies over the years 1994-2002. Highest efficiencies around 96% are achievable with line commutated and transformerless inverter topologies. Improved switching devices and also the trend towards the use of higher DC input voltages have lead to significant efficiency improvements over the last 8 years. However, recent slight efficiency drops are noticeable with some topology types and suggest that PV inverter efficiencies may decrease in order to reduce cost. Fig. 9 gives a more detailed overview on the maximum efficiencies for the different PV inverters types.

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G. Prices Prices for single-phase PV inverters have significantly

decreased during the last 8 years and the trend is expected to

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of 1:4 between transformerless and transformer topologies, whereas in the 2002 survey the ratio is approximately 1:l. Topologies with high frequency transformer have only recently appeared on the European market and seem to have increasing market share. However, the market share information has to be considered carefully and can also not only be viewed from a pure technological perspective. The main players on the European PV inverter market are SMA, Fronius, Sputnik, Sun Power and Siemens, with SMA dominating the market with over 50% market share. Aspects such as fast replacement service or flexible system configuration (for example optional data logging), price as well as efficiencies may influence consumers choices [20].

111. INTERNATIONAL STANDARDS

International standards for single phase grid connected PV inverter systems are still very much at an embryonic stage. PV inverter technology has changed rapidly and many inverter topologies have been developed to explore the most effective and efficient system configuration and the most cost effective design. So the inverter itself has in many cases dictated the sys- tem configuration and also determined important issues such as whether the array is earthed or non earthed. All this inverter de- velopment has taken place at a time when very few standards have existed, so designers have had little restriction in their de- sign imagination. Standards are generally developed after the technology. In the PV inverter area it has been particularly dif- ficult because the technology has been changing rapidly. The topologies have been developing and the PV voltage has been rising rapidly. There is now an urgent need for a range of stan- dards for PV grid connect systems and these will have impor- tant consequences for inverter design. The most important is- sues for standards in this area may be categorized as:

AC issues related to the inverter output, grid interface, pro- tection and safety and possible high AC fault currents on the PV array supplied from the grid. DC issues related to array safety and protection, and common mode voltages impressed on the PV array due to

The AC issues are generally covered by grid interface style standards. The DC issues will be covered by array standards and general wiring standards. The last group of issues particu- larly relate to transformerless topologies. These issues need to be covered by a combination of inverter and array standards.

High common mode voltages impressed on PV arrays fre- quently exist with transformerless inverter topologies where: the PV array does not have a center tapped earth connection or where the centre point of the DC filter capacitors of the inverter input is not earthed. These inverter topologies may cause high common mode voltages to be impressed on the array. This is a potential safety issue because capacitive coupling to a person in contact with the insulated surface of the PV array may result in significant current flowing through the body. The effect is more severe when the common mode voltages are at the

the topology and earthing of the inverter.

switching frequency, allowing a lower impedance coupling. The current may not in itself cause death but because arrays are typically on elevated structures, a reflex reaction may result in a serious fall. Another potential problem relates to high frequency induced currents which may flow in a building structure.

Italy and the United Kingdom do not allow transformerless inverters probably partly due the problem of common mode voltages and also because of the issue of non-isolation of the grid from the PV array and associated wiring.

An overview of standards and current projects relating to arrays, inverters and grid connection in the IEC arena follows.

Standards associated with the grid interface are relatively well advanced with individual countries having standards or guidelines in place. They cover grid protection and anti-islanding issues, basic harmonic requirements, power factor and automatic re-connection I synchronising require- ments. There is no agreement on harmonization of all of the requirements but the documents are not greatly different except in the area of islanding protection. IEC have a project to draft a document on Islanding prevention measures for power conditioners used in grid connected photovoltaic (PV) power generation systems but it is proving difficult to reach a uniform agreement.

The IEC is also working on a draft document No 62109 on requirements for inverters and charge controllers which is derived from the US, UL 1741 and the IEEE 929 documents. This document contains basic safety requirements for the inverter itself but does not address issues of high frequency effects impressed on the array.

Before inverter standards are defined completely there is a need to resolve safety and protection requirements including isolatiodearthing of the PV array. In this area there is a severe lack of standards both within countries and internationally.

The IEC TC64 committee is working on a draft of the IEC 60364-7-Requirements for special installations or locations Section 712 : Photovoltaic power supply systems. This document has been voted on by national committees as a Final Draft International Standard but the results of the vote are not known at the time of writing. This document deals only with extremely basic wiring of the array.

There is still an urgent need for a standard which ad- dresses issues related to: protection of the array (over current protection, bypass diodes); earthed versus unearthed arrays protection and safety requirements; warning signs; access arrangements etc. IEC currently have a project No 62234 Safety guidelines for grid connected photovoltaic (PV) systems mounted on buildings which is aimed at covering these issues but the document is still in a very early stage of development.

In the US the National Electricity Code (section 690) has included requirements for DC arrays which cover earthing and protection. This standard has been available since the mid 1990s but does not allow for an unearthed array, which

1999

V. REFERENCES . @ ~ u i t i stnng inverters

sen mmmutated string mverters wlthout tranformer (< 3 kw)

Self commutated string inverters with tranformer (c 2 kW)

Central self commutated inverters with transformer (> 1 kW)

Central line commutated Inverters (> 1 kw)

1999 1995 2000 Fig. 12. PV inverter development trends (darker and larger &as indicate

increasing importance).

is the general practice in many other parts of the world, particularly in Europe. The Netherlands has had a basic guideline document [21] since 1997 which covers both the grid interface and some DC issues but this is by no means a comprehensive standard. The United Kingdom has recently published a guideline document for array issues E221 and Australia is currently working on a draft document in this area.

In the area of modules themselves the IEC has a well developed project No 61730 which classifies PV modules including an equivalent class I1 module. This project will be presented for voting late in 2002 and when available will resolve many problems related to safety of the modules within the array and simplify the safety requirements for the whole array particularly at voltages above safe touch levels.

Many of these standards under development do not directly relate to inverters but will have important implications for inverter design. Issues of earthing, inverter isolation, insulation monitoring and fault protection systems will affect topology choices and place extra requirements on inverter design. It is extremely important that standards are implemented quickly to promote the safe use of PV systems and implemented carefully on a functional basis so as not to restrict design innovation.

IV. CONCLUSIONS

Fig. 10 attempts to display the development of PV inverter technology with larger and darker areas indicating increasing importance of the specific inverter type. Although the string and multi-string concept has established itself as a popular PV system concept there is no obvious trend noticeable towards a particular topology: The market share ratio of transformerless inverters versus inverters with transformer has remained con- stant over the last two years at 1: 1. Amongst the inverters with transformer, line frequency transformer topologies are far more common, but the number of manufacturers offering inverters with high frequency transformer has increased within the last three years and their market share is expected to rise.

Developments in the area of standards, particularly in the re- quirements for safety in PV arrays will in the future affect de- cisions on preferred topologies.

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